The present invention relates to an apparatus and method for molding a video disc and more particularly to such an apparatus and method which minimizes the adverse effects of defects in the mold on the quality of the picture produced by the disc.
A video disc in general is a flat, circular disc of a plastic material, having recorded information in the form of a surface relief pattern arranged in a spiral path on the surfaces of the disc. In order to achieve compatibility of the playback of the information on the disc and the display of such information on a television screen, it is necessary to provide the disc with blanking intervals periodically along the information track. These blanking intervals are produced at positions where the electron beam in the television tube must retrace either from the end of one line to the beginning of the next line of the picture (horizontal blanking), or from the bottom of the screen to the top (vertical blanking). The speed of the information pickup mechanism, such as a stylus or laser beam, along the information track is often such that the blanking intervals become aligned along radial lines which are uniformly angularly spaced around the surface of the disc. These lines are generally referred to as "sync lines".
Video discs are generally made in a mold press which includes a pair of opposed mold members having mating mold cavities in their opposed surfaces and at least one of which is movable toward and away from the other. Each of the mold plates has a channel extending therethrough adjacent to and extending across its surface which contains the mold cavity. These channels receive a flow of a temperature control medium, such as steam for heating or water for cooling, to control the temperature of the mold cavity. The channel is generally in the form of concentric, circular grooves connected together by radially extending grooves. A stamper is mounted across the mold cavity of each mold plate. A stamper is a thin metal plate having on the surface thereof the negative of the surface relief pattern to be molded on the surface of the disc. If the disc is being molded in a compression molding press, such as described in U.S. Pat. No. 4,327,047 to M. L. McNeely, issued Apr. 27, 1982, entitled "Method for Producing Disc Records Having Molded-in Center Holes", a preform of the plastic material is placed between the heated mold plates which are brought together until the stampers on both mold plates contact the preform so as to heat the preform. As the mold plates are moved further together to the closed condition of the mold, the preform is compressed and the material of the preform is caused to flow radially outwardly to completely fill the mold cavity when the mold plates reach their closed position. The mold plates are then cooled to harden the plastic material and form a rigid disc. The disc has impressed in its surfaces the surface relief pattern on the surfaces of the stamper. If the disc is being molded by an injection molding press, such as described in U.S. Pat. No. 3,989,436 to M. L. McNeely, issued Nov. 2, 1976, entitled "Apparatus for Producing Injection Molded and Generally Appertured Disc Records" the mold plates are brought together to their closed position and the molten plastic material is injected into the mold cavity formed between the mold plates. After the mold cavity is completely filled with the plastic, the mold plates are cooled to harden the plastic and form a rigid disc. The disc has impressed in its surfaces the surface relief pattern on the surfaces of the stampers.
A problem that has arisen in using these techniques for molding video discs is that because of the thinness of the stampers, imperfections in the surfaces of the mold plates can pass through the stampers and result in imperfections in the surfaces of the disc which adversely affect the quality of the video information during the disc playback. For example, it has been found that the radially extending grooves which interconnect the concentric grooves in the mold plates can cause imperfections in the surfaces of the disc which extend across the turns of the spiral path of the surface relief patterns and thus interfere with the playback of the disc. Although it would be most desirable to correct or eliminate such imperfections, there are times when such is either not possible or too expensive to accomplish. Therefore, it would be desirable to be able to mold discs using such mold plates without causing adverse imperfections in the disc.